Can I also add that the intention should be determined by the tone the person uses when asking. If they are asking it in a light hearted open way, they genuinely want to know what you are doing (at the moment or a later point in time).

If there appears to be suspicion in their tone, they may be implying that you are "up to" something you shouldn't be and should stop.

A common phrase is "are you up to mischief?” which means "are doing something naughty, you shouldn't be"?

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